Xeroprinting processes have been known for sometime; see, for example, U.S. Pat. Nos. 2,576,047; 3,271,146 and 3,615,128. Typically, an insulative image is formed on a conductive substrate to provide a xeroprinting master. In use, the master is repetitively charged and toned to create toner images which are transferred to receiving sheets. The xeroprinting apparatus itself is quite simple, requiring only charging, toning and transfer stations. It is capable of running at high speed. Multicolor xeroprinting apparatus has also been suggested in the literature with 3 or 4 drums, each taking a color separation xeroprinting master.
Many approaches to making xeroprinting masters have been suggested in the literature, most of them electro-photographic. In such an electro-photographic method of making a traditional xeroprinting master, a toner image is produced on a photoconductor and transferred to a conductive surface. For example, electro-photographic copiers and printers can be used to make xeroprinting masters using conductive substrates as copy sheets with an insulating toner. Alternatively, masters can be made electrophotographically by fusing a toner image to the surface of a photoconductor. In this case, the toned image acts as an optical mask, and an additional blanket exposure through the mask is required to make each print from the master.
It would be desirable to use improved and less expensive materials for the masters. It is also desirable to use less costly, simpler and presently available printing apparatus for making masters from electronic input.
Japanese Kokai 61/135781 published 23 June 1986 suggests forming a xeroprinting master by the application of either heat or pressure to the backside of an insulating sheet placed on a conductive paper. A thermal printer is used for imagewise application of heat.
U.S. Pat. No. 4,465,749 shows a method of amplifying weak electrostatic images, which method is described with respect to FIG. 1 herein. According to that method a weak electrostatic image on a photoconductor is amplified by applying a charge injection toner to form a faint toner image. The toner image is fused and the photoconductor is recharged. Charges are injected in the dark through the faint toner image to form a greatly amplified electrostatic image which can also be toned. The charge injecting toner thus has the interesting characteristic of essentially turning conductive an otherwise insulating material. The process as described in that patent requires a fusing step even if liquid developers are used to apply the charge injection toner. Example 7 of that patent suggests that the fused toner image can be used as a xeroprinting master and in fact a number of xeroprinting copies were made from it. Although the patent is directed primarily to amplifying weak electrostatic images created by optical exposure, Example 12 suggests applying the charge injection toner electrophoretically and then fusing to create a variation in charge holding ability.
U.S. Pat. No. 3,271,146 suggests use of an asymmetrical photoconductor, zinc oxide, for xeroprinting. A master is made electrophotographically with negative charge, and xeroprinting is done with positive charge. Because zinc oxide will not accept positive charge, no blanket exposure is necessary in xeroprinting.